Abs get a lot of work in any exercise required to keep your torso stable and squats fall into that category. Unfortunately you probably won't see any real ab development with just bodyweight squats, so you might throw in planks, like VBM said, and crunches if you like doing them. If you ever decide to progress to a weight lifting program and start doing weighted squats, overhead presses, and deadlifts then that will be the only ab work you'll need.